The phenomenon known as "sheeting" has continued to cause problems in the use of fluidized beds for olefin polymerization. The incidence of sheeting has been correlated with the presence of electrostatic charges in the reactors. See Fulks et al U.S. Pat. No. 4,532,311.
In U.S. Pat. Nos. 5,321,107 and 5,340,786, water is used as a component of a catalyst for olefin polymerization; the water may be added to the other catalyst components in a hydrocarbon medium. No effect on static in the reactor is reported. See also U.S. Pat. No. 3,342,099 and EPA0308177. In WO 95/13305, water is used to modify catalyst, resulting in increased melt index. See also WO 95/13306.
In European Patent application EPA 0 328 348, water is added continuously as a function of the continuous addition of aluminum as part of a catalyst system, to obtain a desired product flowability. The water addition has no relation to static charge, and no mention is made of static control.
In U.S. Pat. No. 5,432,242, water is used as a volatile metallocene catalyst kill agent for polymer discharged from a high pressure reactor. Water is returned with the recovered monomer to the reactor and is consumed by TMA to form aluminoxane. The monomer and comonomer may also contain water (col 14, line 45) In the flow sheet, these feeds enter the reactor with monomer recovered from the discharge. Catalyst productivity was said to be doubled by using pure water (Example 9) as compared to a mixture of water and glycol-mono-oleate. Very high productivities are shown in Example 15 using water. See also U.S. Pat. No. 5,442,019. In U.S. Pat. No. 5,543,478, steam is used as a deactivator.
In Goode's U.S. Pat. No. 4,803,251, agents are added to a gas phase polymerization zone to drive the static negative or positive toward neutral. Static was measured at a point prone to sheeting. The text of the patent discusses levels up to 25 ppmv on an ethylene feed basis. Water is not claimed or mentioned at this level. The claims are for titanium or vanadium based catalysts with alkyl aluminum cocatalyst. Positive drivers claimed are alcohols, nitric oxide and oxygen. Negative drivers are ketones.
Chirillo et al, in U.S. Pat. No. 4,855,370, disclose a method to reduce sheeting during polymerization of alpha olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalyst together with alkyl aluminum cocatalysts which comprises introducing water into the reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts. Variations include humidifying a nitrogen stream with water and passing it into the reactor, and water content entering reactor is less than one part per million by volume based on the ethylene feed. At column 7, lines 26-29, it is said that the water fed "can generally range in an amount of 0.1 to 2 ppm based on ethylene feed."
U.S. Pat. No. 5,385,991 discloses a process which obviates static and sheet formation on startup by moistening the seed bed before charging. No further additions of water are mentioned, and static is not monitored.
In EP 604,990-B1 and U.S. Pat. No. 5,638,581, monitoring of current associated with static buildup is stated to be of value in controlling a polymerization reaction. Ali, in U.S. Pat. No. 5,688,865, describe a tandem two-reactor process with a high molecular weight component made in the first reactor and a low molecular weight component made in the second reactor, with a Ziegler-Natta catalyst comprising a transition metal compound as a primary catalysts component and an organometallic compound or metal hydride as reducing cocatalyst, a small amount (0.1 to 0.6 ppmv) of water is added to pressure taps of the first reactor to prevent fouling and presumed sheeting at the pressure taps. Titanium based catalyst and triethylaluminum are the only catalyst and cocatalysts mentioned
Antistatic agents said to be useful in preventing the buildup of electrostatic charges in a polymerization system are identified in European Patent Application 0 229 368 A2. The reader may be interested in the operation of the polymerization process with reference to dew point in U.S. Pat. No. 5,453,471 and to the introduction of liquid component in U.S. Pat. No. 5,834,571.